A Replication of Paleoclimate Reconstruction on Variability in the South American Summer Monsoon Over the Last 6 ka from Stable Isotope Analysis on a Speleothem from the Central Peruvian Andes

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Oxygen and carbon isotopes contained in speleothems are excellent high-resolution proxy archives for reconstructing terrestrial paleoclimates. Because speleothems record variations in δ18O and δ13C values, they are valuable multi-proxy climate archives. Therefore, speleothems can provide important information on the paleoclimate in areas such as tropical South America, where previous studies have shown the climate to be quite variable on centennial and orbital scales. These studies have shown that austral summer insolation controls climate on an orbital scale, and Northern Hemisphere temperatures, tropical Atlantic sea surface temperatures, and the El Niño-Southern Oscillation (ENSO) and state of the tropical Pacific control climate on a centennial scale. A previous study by Kanner et al. (2013; QSR 75:1-10), investigated two speleothems from Huagapo Cave in the central Peruvian Andes to study variability of the South American Summer Monsoon (SASM) over the last seven millennia. The authors developed a continuous record of δ18O values from 50 to 7150 years BP, and found that precipitation amount is the greatest controller of variability in δ18O values in speleothems from Huagapo Cave during the last ~7 ka. However, because δ18O and/or δ13C values in speleothems can be altered to not directly represent their primary environmental signals by kinetic processes (deposition occurring outside of isotopic equilibrium), and vadose processes, such as water being evaporated at or near the Earth’s surface, a Replication Test needs to be performed in order to determine if the cave was in isotopic equilibrium during deposition. A recent expedition, from the summer of 2017, to Huagapo Cave obtained another speleothem in order to perform a Replication Test on the δ18O values obtained from the speleothems used in the Kanner et al. (2013) study and then to create a record of δ13C values for the central Peruvian Andes. U/Th dating was used to get an age range for the speleothem, which was found to range in age from 583 ±4 to 6036 ±21 years before present. About 300 samples were collected, between 0.080 mg to 0.090 mg, every 1 mm along the growth axis of the speleothem using a micromill and were analyzed for stable isotopes. The δ18O and δ13C values were plotted versus the age of the speleothem, and the δ18O graph was compared to the δ18O graph from the Kanner et al. (2013) study. Correlation between the replicate speleothems from Huagapo Cave was poor (R values were: δ18O = -0.16 and δ13C = 0.5), which suggests the climate record developed by Kanner et al. (2013) may not be robust. This study highlights the necessity to replicate samples before reconstructing climate.